Air gap-insulated, double-walled exhaust pipes have been increasingly used especially in exhaust systems of motor vehicles, primarily for the area between the internal combustion engine or its exhaust manifold, which collects the exhaust gases from a plurality of cylinders, and an exhaust gas catalytic converter; the latter normally has one or more exhaust gas treatment bodies of large inner surface, through which bodies exhaust gas can flow. The treatment bodies are coated with a catalytically active substance, wherein the exhaust gas treatment bodies are mounted in a sheet-metal housing. Air gap-insulated exhaust pipes bring about a reduction in the release of heat by the exhaust gases to the environment, so that the exhaust gas flows into the exhaust gas catalytic converter at a higher temperature than in the case of a single-walled exhaust pipe. This is significant especially during the warm-up period of the internal combustion engine, because the exhaust gas treatment body will thus rapidly reach its operating temperature. In addition, a relatively thin-walled inner pipe made of a high-temperature-resistant material and an outer pipe of greater wall thickness made of a less expensive material can advantageously be used.
Since the inner pipe of air gap-insulated exhaust pipes reaches considerably higher operating temperatures than the outer pipe, and it frequently consists of a material different from that of the outer pipe, there will be differences in thermal expansion between the inner pipe and the outer pipe during operation. Depending on the length of the air gap-insulated exhaust pipes and the temperature difference between the inner pipe and the outer pipe, there will be differences in length between the inner pipe and the outer pipe, which may easily amount to a few mm. Such differences in length must be compensated, and the provision of a sliding fit between the inner pipe and the outer pipe at one end of the exhaust pipe has hitherto been a common practice. However, particularly if the exhaust pipe has a bend, problems will arise in this simple design, because some areas of the inner pipe will come very close to the outer pipe during heating, which entails the risk of noise generation due to metallic impacts.
The idea of rigidly connecting the inner pipe at both ends to the outer pipe and of providing a sliding fit between two inner pipe sections somewhere between the ends of the exhaust pipe has already been proposed. However, such exhaust pipes have not yet been able to be manufactured economically if a radial mounting of the inner pipe in the outer pipe is to be present somewhere between the ends of the exhaust pipe. Such a radial mounting is especially advantageous in the case of relatively long or bent exhaust pipes. In such cases, the only possibility has hitherto been to cut through the double-walled exhaust pipe, bent to its final shape, at right angles at a point, and to subsequently prepare the sliding fit there between the two inner pipe sections, as well as a radial mounting of the inner pipe in the outer pipe, after which the exhaust pipe was again welded together at the point of separation. Thus, the sliding fit and the radial mounting are practically inherently located at the same point of the length of the exhaust pipe. The described manner of manufacture is cumbersome and expensive.